1. Field of the Invention
The present invention relates to a capsule toner and a method of manufacturing the capsule toner.
2. Description of the Related Art
In an image forming apparatus employing an electrophotography, a surface of an image carrier is uniformly charged by a charging section (charging step), the surface of the image carrier is exposed by an exposure section to dissipate an electric charge of an exposed part so that an electrostatic latent image is formed on the surface of the image carrier (exposure step). Subsequently, a toner composed of fine colored powder having an electric charge is adhered to the electrostatic latent image to be visualized (developing step), and thus obtained visible image is transferred to a recording medium such as paper (transfer step). Further, the visible image is fixed on the recording medium by a fixing section by applying heat and pressure, or other fixing method (fixing step). Through the steps as described above, the image is formed on the recording medium. In addition, cleaning of the image carrier is performed for removing the toner remaining on the surface (residual toner) of the image carrier without being transferred to the recording medium (cleaning step).
A toner to be used for such image formation is necessary to include a function required not only for the developing step but also for each step of the transfer step, the fixing step, and the cleaning step.
Examples of a fixing method of a toner include a heating fixing method of fixing a toner on a recording medium by heating and melting, and a pressure fixing method of fixing a toner on a recording medium by plastically deforming it with pressure. In the heating fixing method, in consideration of the simplification of a fixing device, image quality after fixation and the like, a heat roll fixing method using a heat roll as a heating medium to heat and melt a toner has often been used.
In the heat fixing method, a toner must be melted at as low temperature as possible and fixed to a recording medium. In recent years, requirement to low temperature fixability of a toner is increased particularly from the standpoint of energy saving. The requirement has been responded by decreasing a molecular weight of a binder resin contained in a toner and by adding a release agent to a toner, thereby decreasing a softening temperature of a toner.
However, those methods have the effect in low temperature fixability, but have the problem of decrease in blocking resistance that where a toner is allowed to stand under high temperature, the toner gets soft by heat and is liable to become massed together.
To overcome the problem, Japanese Unexamined Patent Publication JP-A 2005-266565 discloses a capsule toner comprising toner base particles containing a crystalline polyester resin, and a shell layer containing a noncrystalline resin (amorphous polymer). When the shell layer is formed on the surfaces of the toner base particles as in the capsule toner disclosed in JP-A 2005-266565, blocking resistance is improved, and the toner is difficult to become massed together under high temperature.
However, the toner disclosed in JP-A 2005-266565 does not consider complex viscosity of a resin contained in the shell layer. As a result, even in the case of using different kinds of plural resins as a resin constituting the shell layer, there is high possibility that complex viscosities of resins constituting the shell layer are the same level. Thus, a toner having a shell layer constituted of resins having the same level of complex viscosities has the problem that low temperature fixability and hot offset resistance are decreased.